1. Field
This disclosure relates generally to electronic circuits, and, more specifically but not exclusively, to differential amplifiers.
2. Description
Differential amplifiers are ubiquitous building blocks in many electronic systems. In a differential amplifier, relevant information is represented by a difference between two signals. The differential amplifier amplifies the difference between two signals, but rejects common components (“common mode”) of the two signals. Because of this characteristic, a differential amplifier is insensitive to power supply noise and device process variations so long as there is a good match between major devices. Mismatch between the major devices will cause the performance of a differential amplifier to degrade. In practice, however, an ideal match between the major devices is very difficult to achieve, if not impossible, because of random process variations.
In a differential amplifier, common mode voltages of both input signals and output signals may be represented by the average of two input signals and two output signals, respectively. The common mode voltage range is limited by two power rail voltages, supply voltage (“Vcc”) and substrate voltage (“Vss”). In reality, the common mode voltage can rarely shift freely from one power rail voltage to another power rail voltage (“rail-to-rail”) because of such factors as voltage headroom consumed by other devices (e.g., the current source and the source coupled devices). A limited common mode voltage range in turn limits applications of a differential amplifier. For example, when a differential amplifier is used as a sense amplifier, it is desirable for the amplifier to have minimum variations of speed and gain over the rail-to-rail input common mode range.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present application, there is a need to have a wide common mode amplifier with low power consumption and little performance degradation against random process variations among devices.